Phenolic molding compositions have been available for many years and generally consist of phenol-formaldehyde resin blended with various fillers. The molding compositions are prepared by blending a one-stage or two-stage phenol-formaldehyde resin with fillers and then working the material between differential hot rolls in order to soften the resin and obtain a more uniform blend of the constitutents. The composition is then cooled, crushed, and screened for some degree of uniformity of particle size. Unfortunately, such molding compositions are dusty, and lack uniform size.
The dust component remaining in the processed material is also very undesirable. Such dust component tends to cause soiling of the various machines and devices that are used; it is unpleasant to service personnel and may even constitute a danger to health. Moreover, there is a danger of dust explosions. Additionally, the time of contact with the hot rolls is critical, an the process is costly because of the many steps to the final molding composition.
It is further known from the manufacture of thermoplastics to produce granules of uniform size in continuous operation with the processing of the material in a screw extruder. The synthetic plastic after having been melted and homogenized in a screw extruder is pressed out through a perforated plate and then immediately granulated by means of a suitable cutting device cutting the strands of material as they emerge from the perforated plate.
It has also been attempted to granulate thermosetting plastic material immediately following the processing of material in a screw extruder in a manner similar to that used in the processing of thermoplastics. In view of the present great economic importance of synthetic thermosetting plastic materials, it is readily understandable that the industry is very much interested in the development of a process for granulating synthetic thermosetting plastic material in the same continuous, convenient and economic manner in which synthetic thermoplastics can be granulated.
However, the methods heretofore suggested for the purpose are not a truly satisfactory solution of the problem. There are several difficulties inherent in the properties of thermosetting material. The flow resistance offered by the perforated discharge plate is too high for the highly viscous thermosetting plastic material. As a result, there is a heavy load built up at the extruder and additional heating of the material. Such additional heating of the material causes a progressive and unacceptable hardening of the plastic material.
Thermosetting plastic material which is subjected to a too strong thermal loading results in an end product unsuitable for further processing. Moreover, the flow of the material within the extruder becomes irregular; there is a tendancy of forming regions of stagnation within the extruder casings and the orifices or perforations in the perforated discharge plate tend to become clogged. It becomes necessary to stop the extruder after a comparatively short running time. This condition may be corrected, however. Expensive equipement is necessary to correct this problem.